Field of the Invention
The present invention relates to an organic light-emitting diode (OLED) display having a multi-layer stack structure. More particularly, the present invention relates to an OLED display having a multi-layer stack structure, which improves a screen abnormality phenomenon in the outskirt part of a panel by preventing a direct contact between a charge generation layer disposed between stack structures and a cathode electrode disposed to cover the stack structures.
Discussion of the Related Art
A variety of types of flat display devices to replace a bulky cathode ray tube (CRT) are recently developed. The flat display devices include a liquid crystal display (LCD), a plasma display panel (PDP), an electrophoretic display (EPD), and an organic light-emitting display (OLED). The OLED display of the flat display devices is a self-light emission device emitting light itself and is advantageous in terms of high response speed, high emission efficiency, high brightness, and a great viewing angle.
FIG. 1 is a diagram showing the structure of an OLED. As shown in FIG. 1, the OLED includes an organic electric field light-emitting compound layer configured to perform electric field light emission and a cathode electrode and an anode electrode disposed with the organic electric field light-emitting compound layer interposed therebetween. The organic electric field light-emitting compound layer includes a light emission layer EML and may further include a hole injection layer HIL, a hole transport layer HTL, an electron transport layer ETL, and an electron injection layer EIL.
The OLED emits light due to energy from excitons which are formed in an excitation process when holes and electrons injected into the anode electrode and the cathode electrode are recombined in the light emission layer EML. The OLED display displays an image by electrically controlling the amount of light generated from the light emission layer EML of an OLED, such as that of FIG. 1.
An OLED display using the characteristics of an OLED includes a passive matrix type OLED display and an active matrix type OLED display.
The active matrix type OLED display displays an image by controlling an electric current flowing into an OLED using a thin film transistor (hereinafter referred to as a “TFT”).
FIG. 2 is an example of an equivalent circuit showing the structure of a single pixel in an active matrix type OLED display. FIG. 3 is a plan view showing the structure of a single pixel in the active matrix type OLED display. FIG. 4 is a cross-sectional view taken alone line I-I′ of FIG. 3 and shows the structure of the active matrix type OLED display.
Referring to FIGS. 2 to 4, the active matrix OLED display includes a switching TFT ST, a driving TFT DT connected to the switching TFT, and an OLED connected to the driving TFT DT. The TFT of FIG. 4 has been illustrated as being a TFT of a bottom gate method, but is not limited thereto. For example, the TFT may be a TFT having a different structure, such as a top gate method.
The switching TFT ST is formed at a portion in which a scan line SL and a data line DL intersect. The switching TFT ST functions to select a pixel. The switching TFT ST includes a gate electrode SG, a semiconductor layer SA, a source electrode SS, and a drain electrode SD branched from the scan line SL. Furthermore, the driving TFT DT functions to drive the OLED of a pixel selected by the switching TFT ST. The driving TFT DT includes a gate electrode DG connected to the drain electrode SD of the switching TFT ST, a semiconductor layer DA, a source electrode DS connected to a driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving TFT DT is connected to the anode electrode ANO of the OLED. An organic light-emitting layer OLE is interposed between the anode electrode ANO and a cathode electrode CAT. The cathode electrode CAT is connected to a ground voltage VSS. An auxiliary capacitor Cst is disposed between the gate electrode DG of the driving TFT DT and the driving current line VDD or between the gate electrode DG of the driving TFT DT and the drain electrode DD of the driving TFT DT.
More specifically, the gate electrodes SG and DG of the switching TFT ST and the driving TFT DT are formed on the substrate SUB of the active matrix OLED display. Furthermore, a gate insulating layer GI is covered on the gate electrodes SG and DG. Semiconductor layers SA and DA are formed in part of the gate insulating layer GI overlapping with the gate electrodes SG and DG. The source electrodes SS and DS and the drain electrodes SD and DD are formed on the semiconductor layers SA and DA at specific intervals and are configured to face each other. The drain electrode SD of the switching TFT ST comes in contact with the gate electrode DG of the driving TFT DT through a contact hole formed in the gate insulating layer GI. A passivation layer PAS covering the switching TFT ST and the driving TFT DT configured as described above is coated on the entire surface.
The substrate in which the thin film transistors DT and DT have been formed does not have a flat surface due to several elements as described above and has many steps. The organic light-emitting layer OLE is able to emit constant and uniform light only when it is formed on a flat surface. In order to make flat a surface of the substrate, an overcoat layer OC is coated on the entire surface of the substrate.
Furthermore, the anode electrode ANO of the OLED is formed on the overcoat layer OC. The anode electrode ANO is connected to the drain electrode DD of the driving TFT DT through a hole formed in the overcoat layer OC and the passivation layer PAS.
The switching TFT ST and the driving TFT DT are formed over the substrate in which the anode electrode ANO has been formed in order to define a pixel area. Furthermore, a bank BANK is formed on an area in which various lines DL, SL, and VDD have been formed. The anode electrode ANO exposed by the bank BANK becomes a light-emitting area. The organic light-emitting layer OLE is formed on the anode electrode ANO exposed by the bank BANK. The cathode electrode CAT is formed on the organic light-emitting layer OLE.
In the case of the OLED display having a top emission type and implementing full colors as shown in FIG. 4, the anode electrode ANO is formed of a reflection electrode. Furthermore, the organic light-emitting layer OLE may be made of an organic substance generating any one of red, green, and blue. Furthermore, the cathode electrode CAT may be coated on the entire surface of the substrate. For example, the organic light-emitting layer OLE may be made of an organic substance generating white light. In this case, the organic light-emitting layer OLE and the cathode electrode CAT may be coated on the entire surface of the substrate. Furthermore, a color filter may be formed on the organic light-emitting layer OLE or the cathode electrode CAT.
In the case of an OLED display having a bottom emission type and implementing full colors, a color filter is further formed between the overcoat layer OC and the passivation layer PAS. The anode electrode ANO may include a transparent conductive substance. In this case, the organic light-emitting layer OLE may be made of an organic substance generating white light. Furthermore, the organic light-emitting layer OLE and the cathode electrode CAT may be coated on the entire surface of the substrate. Accordingly, the OLED display is completed.
Recently, active research is carried out in order to improve current efficiency of an OLED display and to increase the lifespan of a light-emitting device. For example, research is carried out in order to provide an OLED display having high brightness using the same consumption power as that of a prior art device or an OLED display having the same brightness as that of a prior art using lower consumption power.